Refrigeration & Air-Conditioning Practice Set-3

May 18, 2021

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1. A certain refrigerating system has a normal operating suction pressure of 10 kg/cm gauge and condensing pressure of about 67 kg/cm. The refrigerant used is

Brine

Carbon dioxide

Freon

Ammonia

2. A reversible engine has ideal thermal efficiency of 30%. When it is used as a refrigerating machine with all other conditions unchanged, the coefficient of performance will be

1.33

2.33

3.33

4.33

3. A bootstrap air cooling system has

Four heat exchangers

Three heat exchangers

Two heat exchangers

One heat exchanger

4. Which of the following statement is correct?

(D) Both (A) and (B)

(B) The increase in air movement increases the evaporation from the human body.

(A) The human body can lose heat even if its temperature is less than the atmospheric temperature.

5. The freezing point of sulphur dioxide is

-75.2°C

-77.7°C

-56.6°C

-135.8°C

6. In a lithium bromide absorption refrigeration system

Ammonia is used as a refrigerant and lithium bromide as an absorbent

Water is used as a refrigerant and lithium bromide as an absorbent

Lithium bromide is used as a refrigerant and water as an absorbent

None of these

7. A thermostatic expansion valve in a refrigeration system

Is suitable only for constant load systems

None of the above

Maintains different temperatures in evaporator in proportion to load

Ensures the evaporator completely filled with refrigerant of the load

8. Freon group of refrigerants are

Non-inflammable and toxic

Nontoxic and non-inflammable

Toxic

Inflammable

9. Which of the following is not a desirable property of a refrigerant?

High risibility with oil

Large latent heat

Low boiling point

Good electrical conductor

10. The domestic refrigerator uses following type of compressor

Piston type reciprocating

Miniature sealed unit

Centrifugal

Axial

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11. Rating of a domestic refrigerator is of the order of

5 tons

0.1 ton

10 tons

40 tons

12. If a heat pump cycle operates between the condenser temperature of +27°C and evaporator temperature of -23°C, then the Carnot COP will be

1.2

0.2

13. The bank of tubes at the back of domestic refrigerator is

Condenser tubes

Refrigerant cooling tubes

Evaporator tubes

Capillary tubes

14. In a flooded evaporator refrigerator, an accumulator at suction of compressor is used to

Detect liquid in vapour

Collect vapours

Collect liquid refrigerant and prevent it from going to compressor

Superheat the vapour

15. Most thermostatic expansion valves are set for a superheat of

5°C

15°C

10°C

20°C

16. Which of the following cycles uses air as the refrigerant?

Stirling

Carnot

Ericson

Bell Coleman

17. The operating pressure for refrigerating units using R-12 as a refrigerant is

15 bar

30 bar

2 bar

8 bar

18. Cooling water is required for following equipment in ammonia absorption plant

Condenser, absorber and separator (rectifier)

Condenser

Absorber

Evaporator

19. Critical temperature is the temperature above which

Water will evaporate

Water will never evaporate

A gas will never liquefy

A gas will immediately liquefy

20. The value of C.O.P in vapour compression cycle is usually

Always more than unity

Always less than unity

Equal to unity

Any one of the above

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21. The capillary tube, as an expansion device, is used in

Domestic refrigerators

Water coolers

Room air conditioners

All of these

22. The general rule for rating refrigeration systems (excepting for CO₂ system) is to approximate following h.p. per ton of refrigeration

2 to 5 h.p. per ton of refrigeration

0.1 to 0.5 h.p. per ton of refrigeration

0.5 to 0.8 h.p. per ton of refrigeration

1 to 2 h.p. per ton of refrigeration

23. The atmospheric air at dry bulb temperature of 15°C enters a heating coil maintained at 40°C. The air leaves the heating coil at 25°C. The bypass factor of the heating coil is

0.6

0.376

0.4

0.67

24. In a refrigeration system, the expansion device is connected between the

Evaporator and compressor

Compressor and condenser

Condenser and receiver

Receiver and evaporator

25. The boiling point of ammonia is

-30°C

-10.5°C

-33.3°C

-77.7°C

26. The C.O.P. of a refrigerator working on a reversed Carnot cycle is (where T₁ = Lowest absolute temperature, and T₂ = Highest absolute temperature)

T₁ / (T₂ - T₁)

(T₁ - T₂)/T₁

(T₂ - T₁)/T₁

T₂/ (T₂ - T₁)

27. The minimum temperature to which water can be cooled in a cooling tower is

Dry bulb temperature of air

Wet bulb temperature of air

Ambient air temperature

Dew point temperature of air

28. The refrigerant after condensation process is cooled below the saturation temperature before throttling. Such a process is called

Super-cooling

None of these

Normal cooling

Sub-cooling or under-cooling

29. The condition of refrigerant after passing through the condenser in a vapour compression system is

Dry saturated vapour

Superheated vapour

Wet vapour

Saturated liquid

30. Highest temperature encountered in refrigeration cycle should be

Near critical temperature of refrigerant

At critical Temperature

Above critical temperature

Much below critical temperature

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31. The vapour compression refrigerator employs the following cycle

Reversed Carnot

Reversed Rankine

Carnot

Rankine

32. On the pressure-enthalpy diagram, condensation and desuperheating is represented by a horizontal line because the process

Takes place at constant pressure

Involves no change in volume

Takes place at constant temperature

Takes place at constant entropy

33. In a refrigeration system, heat absorbed in comparison to heat rejected is

Less

Same

More for small capacity and less for high capacity

34. A human body feels comfortable when the heat produced by the metabolism of human body is equal to the

(B) Heat stored in the human body

(D) Difference of (A) and (B)

(A) Heat dissipated to the surroundings

35. In actual air-conditioning applications for R-12 and R-22, and operating at a condenser temperature of 40° C and an evaporator temperature of 5° C, the heat rejection factor is about

1.25

2.15

5.12

36. For obtaining high COP, the pressure range of compressor should be

Any value

Optimum

High

Low

37. Mass flow ratio of NH₃ in comparison to Freon-12 for same refrigeration load and same temperature limits is of the order of

1: 3

1: 1

1: 9

9: 1

38. The change in evaporator temperature in a refrigeration cycle, as compared to change in condenser temperature, influences the value of C.O.P.

Unpredictable

Less

Equally

39. When the temperature of the surrounding is higher than the temperature of the body, then the heat loss by convection from the body to the surrounding will be

None of these

Zero

Positive

Negative

40. The curved lines on a psychrometric chart indicates

Wet bulb temperature

Dry bulb temperature

Dew point temperature

Relative humidity

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A Demrot is the founder of VideRime Online Learning, a leading engineering website. He did his BE Civil and M.Tech Structure from RGPV University, Bhopal and has been working as an Assistant Professor in a reputed college.

Refrigeration & Air-Conditioning Practice Set-3

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